EQUIPOS DE PREPARACIÓN DE MUESTRAS:
Critical Point Dryers 
FEDELCO,
S.L.
CRITICAL
POINT DRYERS:
The advent of Scanning
Electron Microscopy (SEM) in the study of surface morphology in
biological applications made it imperative that the surface detail of a
specimen was preserved.
The air (evaporative) drying of specimens can cause deformation and
collapse of structures, the primary cause of such damage being the
effects of surface tension. The specimen is subjected to forces, which
are present at the phase boundary, as the liquid evaporates. The most
common specimen medium, water, has a high surface tension to air, by
comparison that for acetone is several times lower. The surface tension
could be reduced by substitution of a liquid with a lower surface
tension with expectations of reduced damage during air-drying.
However, the occurrence of what is known as 'continuity of state'
suggests a drying technique for which the surface tension can be reduced
to zero.
If the temperature of liquefied gas is increased the meniscus becomes
flatter indicating a reduction in the surface tension. If the surface
tension becomes very small the liquid surface becomes very unsteady and
ultimately disappears.
When this 'critical point' is reached, it is possible to pass from
liquid to gas without any abrupt change in state. If a specimen had been
in the liquid it would have experienced a transition to a 'dry' gas
environment without being in contact with a surface, avoiding the
possibility of the damaging effects of surface tension.
Critical Point Drying is so named as it includes, as part of its
process, the occurrence known as the continuity of state for which there
is no apparent difference between the liquid and gas state of a medium,
the surface tension between this interface reducing to zero. This occurs
at a specific temperature and pressure with resulting density, and is
known as the Critical Point. This condition of zero surface tension can
be used to dry Biological Specimens, avoiding the damaging effects of
surface tension.
This is termed Critical Point Drying (C.P.D.) the basis of which are the
classic experiments carried out over 100 years ago during investigations
on the liquefaction of gasses.
